1. Field of the Invention
The present invention relates to a steel wire for gas shielded welding, more particularly a steel wire useful for performing all-position welding of a large gauge steel plate mainly under short circuiting transfer conditions.
2. Description of the Prior Art
Generally, the short circulating transfer type welding is easily performed in all welding positions, and the penetration obtained by the welding is shallow and good bead appearance can be obtained. Thus, this type of welding has been widely used for welding a thin steel sheet.
In recent years, in order to enhance welding efficiency, the tendency towards automatic or semi-automatic welding operations by applying the short circuiting transfer welding method to the multi-layer welding of relatively thick steel plate which hitherto had been welded manually, has increased.
However, when the conventional short circuiting transfer welding method is applied to the multi-layer welding of a thick plate, only a shallow penetration is obtained, and, as a result, satisfactory fusion is not attained and slag inclusion is most likely to take place. Also, in the case of a steel pipe, good back bead formation cannot be obtained when one side back bead is required in the first layer.
In the short circuiting transfer welding, the length of the arc between the electrode wire and the base metal to be welded is controlled by adjustment of the arc voltage, and arc generation short circuiting and transfer of droplet are repeated more than ten times per second. Thus, this welding method has certain periods in which no arc is generated, and the frequency of the droplet transfer is high so that the welding can be easily performed in all positions, overheating of the base metal can be prevented, and the penetration is shallow. The fact that the penetration is shallow is normally advantageous because no fusion loss is caused. However, when the short circuiting transfer welding is applied to a thick plate, it does not give enough heat for melting the base metal, and the melting power of the arc when it is generated is inferior so that defects, such as, incomplete fusion and slag inclusion, result.
Further, in a gas shielded welding, a power source of D.C. constant voltage characteristics is used and a control system by constant speed feeding is adopted.
According to the D.C. constant voltage power source system, if the arc voltage which should be constant increases only slightly due to the surface irregularity and other factors, the welding current instantaneously decreases remarkably when the arc voltage increases so that the fusion rate of the wire is retarded and the space between the base metal and the wire is shortened and the arc voltage returns to its original value.
On the contrary, when the arc voltage gets lower than the constant value, the welding current increases sharply and the fusion rate of the wire increases so that the space between the base metal and the wire is elongated and thus the arc voltage returns to its original value.
Therefore, in the gas shielded welding which is commonly used, it is important to maintain the arc length constant utilizing the self-control ability of the constant voltage power source.
Based on the above principle, it is necessary that the fusion rate of the wire changes corresponding to delicate variations by the welding current depending on the change of the arc length. For this purpose, any abrupt change in the fusion of the wire should be avoided and the fusion should proceed smoothly against any current change.
However, conventional welding wires have the defect that arcs produced by these wires are so unstable that the arcs can not be satisfactorily controlled by the inherent self-control of the arc voltage, and the wires get into the molten pond thereby causing the so-called stick phenomenon, thus resulting in vigorous spattering and deterioration of the bead shape as well as unsatisfactory fusion of the wire.
Under the current tendencies toward the automatization and mechanization of the welding operation, development of welding wires has been strongly demanded in many industrial fields, such as, ship-building, car-manufacturing and building industries.